is this condition true ? yes : no

int num = 5;
if (n > 0) { System.out.println("This is a positive number")
}

if(true) { document.write("True");
} else { document.write("False");
}

if (condition) {	// code
} else {	// code
}

var price = 500;
var money = 100;
if (price > money) { console.log("Not Enough");
} else { console.log("Can Buy");
}

How is written:
if() {
}
"if" is used to do something just IF a condition is true
In parenthesis, you place your condition. The condition should be of type true/false.
Examples:
if(1 + 2 == 3); if(4 > 3); if(variable != 7);
"Note!" If you want to check an equality / inequality use "==" / "!=";
If the condition is true, whatever is between braces is gonna happen;
Connected to an "if" Statment there can also be "else if" or "else"
Examples:
if(variable == 2) { //Code }
else if(variable == 1) { //Code }
else if (variable == 0) { //Code }
else { //Code }
If the first if the condition wasn't true then it will check the other "else if".
If one of them is true the rest won't be checked by the program
And if none of the if/else if are true, the program will execute the code in the else braces.
"Note!" There can only be one "else" in an if statement and how many "else if" you want

if(Boolean_expression) {

if(Boolean_expression) { // Statements will execute if the Boolean expression is true
}

if(Boolean_expression) { // Statements will execute if the Boolean expression is true
}
If the Boolean expression evaluates to true then the block of code inside the if statement will be executed. If not, the first set of code after the end of the if statement (after the closing curly brace) will be executed. 

if somecommand; then # do this if somecommand has an exit code of 0
fi